Gas phase acidity

K has been identified as CFl200I-I from its chemistry the reaction mechanism is insertion [115], Collision-induced dissociation (in a SIFT apparatus, a triple-quadnipole apparatus, a guided-ion beam apparatus, an ICR or a beam-gas collision apparatus) may be used to detemiine ligand-bond energies, isomeric fomis of ions and gas-phase acidities. 

It is possible to detemiine the equilibrium constant, K, for the bimolecular reaction involving gas-phase ions and neutral molecules in the ion source of a mass spectrometer [18]. These measurements have generally focused on tln-ee properties, proton affinity (or gas-phase basicity) [19, 20], gas-phase acidity [H] and solvation enthalpies (and free energies) [22, 23] ... 

Additional gas-phase reactivity data, such as gas-phase acidities of alcohols [41], proton affinities of alcohols and ethers [41], and proton affinities of carbonyl compounds [42] could equally well be described by similar equations. 

Fundamental enthalpies of gas-phase reactions such as proton affinities or gas-phase acidities can be correlated with the values of the Inductive and the polarizability effect. 

This is opposite from the order in solution as revealed by the pK data in water and DMSO shown in Table 4.14. These changes in relative acidity can again be traced to solvation effects. In the gas phase, any substituent effect can be analyzed directly in terms of its stabilizing or destabilizing effect on the anion. Replacement of hydrogen by alkyl substituents normally increases electron density at the site of substitution, but this effect cannot be the dominant one, because it would lead to an ordering of gas-phase acidity opposite to that observed. The dominant effect is believed to be polarizability. The methyl... 

Comparison of the gas-phase acidity of benzoic acids with pAT values of the same compounds in aqueous solution provides some interesting relationships. 

The basicities of amines, ethers, and carbonyl compounds are invariably decreased by fluonnation. 2,2,2-Tnfluoroethylamine (p f = 3.3 [61]) and C 5NH2 = -0.36 [62]) are about 10 times less basic than CH3CH2NH2 and CgH5NH2, respectively, and (CF3)2CHNH2 (p j, = 1 22 [71]) is over 10 times less basic than 1-C3H2NH2. The relative gas-phase acidities in Table 15 illustrate the large effect of fluonnation. Perfluoro-rerf-amines (R )3N and ethers R Rf have no basic character m solution [, 74], and CF3COCF3 is not protonated by superacids [72]. 

SOMe the enhancement in the meta-position is almost as large as in the para-position. The authors go on to show the applicability of op (g) values to certain solution processes, particularly those in non-aqueous solvents, but including the dissociation of thiophenols in 48% ethanol, the results of Bordwell and Andersen80 to which reference has been made earlier (Section III.A.1). A separation of field/inductive and resonance effects is also essayed for the gas-phase acidities of the phenols, and SOMe and S02Me feature in the discussion. There is reference to a oR° value of + 0.07 for SOMe as an unpublished result of Adcock, Bromilow and Taft (cf. 0.00 from Ehrenson and coworkers65 and — 0.07 from Katritzky, Topsom and colleagues128.)... 

Recent studies have found enhanced substituent solvation assisted resonance effects in dipolar non-hydrogen bonding solvents131. For several +R substituents acidities of phenols in DMSO are well correlated with their gas-phase acidities. The substituents include m- and p-SOMe, m- and p-S02Me, m-S02CF3 and m-N02. But there is very considerable enhancement of the effect of p-S02CF3, p-N02 and various other para-substituents in DMSO solution. 

Studies of gas-phase acidities of carbon acids have appeared in the last ten years, including work on sulfones and sulfoxides189. The acidity of substituted methanes increases in the order... 

Since the gas phase acidities of the sulfanes increase with the chain-length the equilibria... 

Gas-phase Acidity The enthalpy for heterolytic cleavage to form an... 

The free energy for this reaction, AGj,cid. is also called the gas-phase acidity. 

Because of this, the thermochemistry of many physical processes can be described in different ways. Thus, the ionization energy of neutral A is the same as the electron affinity of A, the proton affinity of B is also the gas-phase acidity of BH, and the gas-phase acidity of AH is the same as the proton affinity of A. ... 

In principle, the equilibrium approach can be used to measure any of the thermochemical properties listed above. However, in practice, it is most commonly used for the determination of gas-phase acidities, proton affinities, and electron affinities. In addition, equilibrium measurements are used for measuring ion affinities, including halide (F, Cl ) and metal ion (alkali and transition metal) affinities. 

Energy-resolved CID can be used to measure bond dissociation energies directly, and therefore is readily applicable for the determination of ion affinities. However, Graul and Squires have also described a method for measuring gas-phase acidities using CID of carboxylates. ° Upon CID, carboxylate ions, RCO2, undergo decarboxylation to form the alkyl anions, R, ... 

Threshold CID can be used to measure the energy required for decarboxylation, in order to determine the enthalpy of formation of R, which can be used to calculate the gas-phase acidity. While nominally straight-forward, the decarboxylation approach is limited to systems that have a bound anion, R, and requires an instrument with the capability of carrying out energy-resolved CID. However, it does have an advantage of being a regiospecific approach. 

Wenthold and co-workers have used this approach to generate the negative ion of 5-dehydro-m-xylylene (Eq. 5.19). ° Using bracketing experiments to determine the gas-phase acidity at the 5-position of m-xylylene, and the kinetic method to determine the triradical electron affinity, the BDE at the 5-position of m-xylylene was measured (Eq. 5.20). 

The relative gas-phase acidities or basicities obtained via equilibria (equation 1) were converted to absolute gas phase acidities,... 

Relationships between Hydration Energies and Gas-Phase Acidities and Basicities... 

Experimental determinations undertaken prior to the discovery of electrospray as a source of ions have shown86 87 that the bond strength of H-bonded complexes XH —A" increases with the gas-phase acidity of XH and the gas-phase basicity of A-. This relation has been examined82 for the special case where A- were a variety of anions produced by electrospray and XH = OH2, on the basis of the hydration energy data (see Table 8) and gas-phase basicities AGj A-) = AG°cid (AH) corresponding to the free energy change for the gas-phase reaction ... 

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